The development of receptors for biologically important anions is emerging as a research area of great importance.1 Pyrophosphate anion (hereinafter “PPi”), in particular, participates in several bioenergetic and metabolic processes,2 such as the synthesis of cyclic AMP as a second messenger from ATP with the concomitant release of PPi and the production of calcium pyro-phosphate dihydrate (CPPD) crystals.3 It is the deposition of CPPD crystals that is frequently detected in patients with osteoarthropathy or pseudogout.3 This diversity of function, both beneficial and otherwise, is why the detection of PPi is the main focus of many research groups today. While PPi analysis such as ion chromatography remains important, there is mounting incentive to find alternative means of analysis, including those based on the use of selective chemosensors.1b,4 Particularly useful would be systems that can recognize PPi in an aqueous solution and signal its presence via various signals (for example, an optical signal, a fluorescent signal and an electrical signal). Until now, very few examples of sensors for PPi in aqueous solution have been reported.5,6 Further, the sensors failed to give satisfactory results.
Suitable PPi sensor should satisfy the following requirements:
(1) High selectivity for the pyrophosphate anion over other anions;
(2) High sensitivity for the pyrophosphate anion over other anions;
(3) Sufficient compatibility to an aqueous system; and
(4) Applicability over a wide pH range or insensitivity to pH change.